Many particulate materials used in industry and in the home contain a substantial amount of dust which tends to become airborne when the material is transferred or transported in bulk or when carried on a conveyor or poured from a bag or other container, often creating an environmental condition hazardous to the health of workers in the vicinity, and/or producing an explosive mixture capable of destroying the facility handling the material. This condition is prevalent in the grain handling or milling industries and in the mining, quarrying and processing of minerals. While the dust in the particulate material is itself a particulate, for the purpose of the description herein, the particulate material of useful or desired size will be referred to as the "particulate material", and the undesirable or smaller particles which can readily become air-borne will be referred to as the "dust", the difference often being merely in the size of the particles, with the smaller particles being the component to be removed from the base particulate material.
In the clay industry, particulate material is prepared for a variety of uses, including oil and grease absorption and animal litters, wherein the clay is heated to drive off the moisture and then ground and graded to obtain a bulk material of substantially uniform particle size. In the processing of this material, it is first crushed and heated to drive off the moisture, and thereafter screened and stored in bulk and/or bagged or otherwise packaged for shipping. As the material passes through these various stages of the operation, dust, i.e. relatively small undesirable particles, is created in the particulate material from friction between the particles and/or contact of the particles with the equipment, thereby rendering the material less suitable for such uses as an oil absorbent or animal litter. The dust and/or useful particles may have an electro-static charge which causes the dust to adhere to the desirable particles, and hence interfere with the separation of the dust from those particles by conventional screening equipment. In the past, screens, cyclones, surfactants, air separators, or other air classification systems have been used or tried, but these systems have either been expensive to install and operate, have required excessive amounts of space in the production operation, or have not produced subtantially dust-free material.